1. Field of the Invention
This invention relates to a method of confirming the validity of a valuable document as protection against counterfeiting, and in particular to a method utilizing a magnetic material whose optical properties change in an applied magnetic field.
2. Description Relative to the Prior Art
The counterfeiting of currency, stocks, bonds, credit cards and other valuable documents essential to the orderly and effective carrying on of business and financial activities is a continuing serious problem. The widespread availability of high quality imaging systems and the increasing technological sophistication of the criminal combine to increase the complexity of combatting all forms of counterfeiting.
Currently, considerable resources are being devoted to the development of devices for incorporation into a document which can be detected to validate the document's authenticity. Holograms, opaque print strips and microprinting are examples of such devices, and their effectiveness depends upon the difficulties involved in counterfeiting them.
It will be noted that while it may be difficult for a criminal to reproduce these countermeasures in his bogus document, generally they are made of readily available materials. With the passage of time, the astute counterfeiter may develop a copying technique and readily obtain the materials needed to counterfeit the document as well as the "anti-counterfeiting" feature itself. The anti-counterfeiting method of the present invention circumvents this problem by use of materials that would be difficult for the typical counterfeiter to duplicate, and for which the sources of supply are limited. This restriction in the availability of the material is a bottleneck through which an aspiring counterfeiter must squeeze, increasing his vulnerability to detection and exposure.